The present invention relates to a projection optical system for projecting a first object pattern onto a second object substrate for device manufacture. More specifically, the present invention relates to a projection optical system which is suitable to superimposingly photoprint a pattern formed on a first object reticle (mask) for semiconductor or liquid crystal display onto a substrate (e.g., wafer, plate, etc.) as a second object.
As integrated circuits become more miniaturized, specifications for higher performance have been required for projection optical systems used for photoprinting a wafer. To improve resolution for projection optical systems, it has been proposed that shorter .lambda. exposure light be used, or to utilize a larger numerical aperture (NA) of the projection optical system.
In recent years, in order to meet the miniaturization of printing patterns, a light source which emits from g-line (436 nm) through i-line (365 nm) has been popularly used. A light source which emits light of shorter wavelength such as excimer laser (KrF: 248 nm; ArF: 193 nm) is about to be used. Consequently, projection optical systems have been proposed which use exposure light of the above-mentioned wavelength to superimposingly photoprint a pattern from a reticle onto a wafer.
In a projection optical system, it is required that both resolution be improved and levels of image distortion be decreased. The image distortion includes not only the distortion caused by the projection optical system (distortional aberration), but also distortion due to a warped wafer onto which an image is photoprinted on the image side. Also, distortion may be caused by a warped reticle onto which a circuit pattern and the like is drawn on the object side.
The miniaturization has been further advanced lately and the demand for decreasing image distortion has also grown greatly.
Conventionally, a so-called "image-side telecentric optical system" has been developed. This system locates a point of an exit pupil of the optical system at a farther point on the image side in order to decrease the impact of a warped wafer on the image distortion.
It has also been proposed to use a so-called "object side telecentric optical system," which locates a point of an entrance pupil of the optical system at a farther point from the object side. There are also proposals in which techniques to locate the entrance pupil of a projection optical system relatively farther from the object surface are introduced. Japanese patents S63-118115, H4-157412, and H5-173065, etc. are some of these examples.
In the above-mentioned Japanese patents, the so-called "bitelecentric projection optical system" having a telecentric object side and a telecentric image side is disclosed.
Nevertheless, the numerical aperture (NA), which affects resolution, is not large enough and aberration correction capability for each of the aberrations, specifically distortion, is not satisfactory for the bitelecentric projection optical system proposed in each of the above patents.